i would REALLY like to know how your installer is going to do what you say he is. i have a CLK also and i have been trying to solve problems with my stereo for awhile. i must say that using an ORION amp with a factory deck is what i would expect a shop to suggest. that amp (i assume not the entry level) will NEVER see its potential because of the signal limitations of the factory unit and speakers (the triangle speaker is made by NOKIA), especially "trying" to get a low level signal to the ORION. are they going to wire directly from the pre-amp board in the deck for the low level signal? do they suggest taking the high level signal from the factory amp, use a line level converter to run the RCAs off of , and connect to the amp? are they aware of the "load" of the speakers and how and where they are wired? keep in mind that there are 4 channels that drive 8 speakers. the factory speakers have VERY odd ohm ratings. i am in the process now of trying to figure out the easiset way to improve the sound in my CLK and believe me, it is NOT easy and very few "experts" have experience with this problem. if you are going to spend big bucks on an ORION, spend more big bucks to do it right or you will be very upset.
i like the idea of free air subs. i have used them in the past and they sound great. the problem is finding them.
i know this is alot of information/opinion, but, really, i would pay to know how this installer is going to do this. if you have more concerns/questions, e-mail me.

trent, the sonys work well in free air?? do you have any vibration problems comming from the deck? is this on a CLK?? i have a civic with 2 12s facing directly into the seats and it hits very hard. a friend with the same car faced them to the rear with no other mods with the seats up and it hits harder! i can put the seats down and i cant notice the difference. thanks for the info..